Fuel cells transform chemical energy to electrical energy by reacting gas in the presence of an electrolyte, electrodes and a catalyst. A catalyst and the electrodes may be platinum or an expensive material. Consequently it is desirable to use as little catalyst and electrode material as possible. Fuel cells are often used to generate electricity in remote locations. Consequently it is highly desirable to construct a fuel cell as compact and as lightweight as possible. To produce a fuel cell that is economically viable as a mass produced product, the process of forming the fuel cells needs to be one that optimizes the cost of materials, the cost of production and the operational performance.
A 1961 British Patent 874,283 describes a micro-porous fuel cell electrode based on un-plasticized polyvinyl chloride. The typical polyvinyl chloride researched was 0.76 mm thick and had very uniform 5 micron pores. Surfaces were metallized by vacuum evaporations of silver or gold. The catalyst layer was applied by electro-deposition or incorporated in a binder. Cells up to 5 kW air hydrogen were formed but were limited to 65.degree. C. by the use of polyvinyl chloride. Polyethylene porous substrates were considered to be usable up to 80.degree. C.
The concept of supporting fragile electrodes with a fiber matrix has been used. Siemens Company used a porous layer of powder embedded into an asbestos membrane. The asbestos membrane provided the mechanical support for the powder electrode.
An etched porous Vycor glass substrate has been sputter coated with tantalum and platinum films to form electrodes. The electrodes formed a fuel cell with a high catalyst utilization. Further research concluded that glass Vycor substrates were impractical and that porous metal electrodes offered no advantage in sputter-depositing the catalysts within the context of a space application fuel cell.
The present inventor used an etched-nuclear-particle-track membrane (such as a Nuclepore filter made by the Nuclepore Corporation, Pleasanton, Calif.) as the substrate, so that the electrode would have the toughness of a plastic film and the exact pore geometry needed for micro-engineering. By using a simple pore model the output of the cells could generally be predicted. The thinnest cell tested was nominally 10 microns thick. It appears that the practical limitations on the cell's thickness are the membrane's strength, fuel diffusion resistance, and cooling capacity. A minimum cell thickness minimizes the cell resistance losses and maximizes the power per unit mass ratio. There are several difficulties with the coating of the nuclear-particle-track-dielectric film with vacuum deposited catalysts and electrodes. A first is that the vacuum deposited films often have poor sticking coefficients to the dielectric films and separate from the film during operation. A second is that the dielectric films are often not capable of operating at the higher temperatures or electrolyte environments. A third is that to form series cell stacks, the metal films need to be thickened with conductive metal films to avoid mechanical damage from the cell contact or high film resistivity. Most of the metal films that are non-corroding conductors and can be deposited at low enough temperatures for the plastic substrates are of comparable expense to the catalyst films, such as gold. The cost of the bulk metal conductors becomes a limiting factor.
Electrolytes that could be advantageously incorporated into the electrodes of the present invention are the perfluorinated ion exchange polymer electrolytes such as Nafion from E. I. DuPont de Nemours. Nafion has been adsorbed into Nuclepore membranes and expanded PTFE matrixes. Perfluorinated ion exchange polymer electrolytes or Proton Exchange Membranes (PEM) are commercially available with expanded PTFE reinforcing from E. I DuPont. The effective conductivity through the Nafion is increased by as much as 20 fold over the original Nafion membranes by being in the Nuclepore membranes. Thus, by structuring the perfluorinated ion exchange polymer electrolyte, the conductivity is enhanced, reducing the amount of polymer electrolyte needed and the gas diffusion simultaneously. The new invention describes this as a collimated electrolyte. Lateral electrolyte ionic conduction, perpendicular to the pore direction, is blocked by the collimating dielectric membrane. The technique of forming fuel cell electrodes on a matrix impregnated with a polymer electrolyte is described in U.S. Pat. No. 4,666,579. The methods of dissolving the Nafion in alcohol are described in U.S Pat. No. 5,084,144.
The current state of the art for PEM fuel cells has been to deposit "platinum inks" onto perfluorinated ion exchange polymer electrolytes as disclosed in U.S. Pat. No. 5,084,144. It has been realized by researchers in the field that the platinum utilizations are now to the point where the remaining components costs dominate the cost of the fuel cells, for example the cost of perfluorinated ion exchange polymers such as Nafion membranes. Membranes as thin as 20 microns have been manufactured and have achieved current densities of 3 amps per square centimeter, driving costs down by getting more power per unit area. Thinner electrolyte films have not been used because of the films being fragile and pinhole defects causing shunting of the electrodes. The apparent fundamental assumption in the general field is that the cost per unit area is relatively fixed due to the frame and gas separator costs. The next considerations are functionality and manufacturability.
Humidity control is an ongoing engineering concern of many fuel cell designs. The fuel cells that do not circulate the electrolytes tend to dehydrate their electrolytes, because they operate hotter than their surroundings. That can lead to the cells operating far from the optimum conditions in the fuel cells. The typical method of re-hydrating the fuel cells is to capture water in the exhaust stream in a colder condenser, and then to humidify the fuel supply gas above the water vapor pressure of the fuel cells with a higher temperature vaporizer. That adds weight and complexity to the operation of the fuel cells. One solution has been to flow water back through a central hole as disclosed in U.S. Pat. No. 5,242,764. That electrolyte recirculation eliminates the need for high differential pressures to control electrolyte water balance, thereby eliminating the need for the electrodes and separator backing to withstand large pressure differences. That permits fuel cells to be far lighter. Part of the electrolyte is mobile. Problems associated with a mobile electrolyte include the need for additional space in the electrolyte above what is needed for fuel cell operation for lateral water diffusion. The added size of that space depends on the distance and diffusion resistance to the central flow through hole. That space could significantly increase the cell's resistance. U.S. Pat. No. 5,242,764 also requires more expensive electrolyte to accommodate the lateral movement of water through it and adds costs and weight to the cell. By having a mobile electrolyte, there could be leakage, depletion and corrosion problems.
Current low temperature fuel cell stacks (roughly below 200.degree. C.) use bipolar stacking of the electrode and gas separation partitions. The partitions need to be electrically conductive and gas impermeable. They often need to survive in the same electrolyte environment as the fuel cell electrodes. The separators usually need to withstand the gas pressure differences between the fuel and the oxidizer gases. Thus the separators are typically mechanically robust. That leads to the need to use bulk metal separators with at least non-corrosive metal exteriors such as graphite, doped diamond, platinum or gold coatings. By making the electrical contact onto those separators in the moist corrosive environment (fuel and oxidizer on either side and contact with electrolyte and product water) there can be corrosion and fuel cell lifetime reductions. By having a large fraction of the fuel cell stack made of metal, the fuel cell stack has a higher liability of catastrophic electrical and explosion failure when a shunt occurs. The shear bulk mass of the gas separators reduces the specific power per unit mass of the fuel cell.
Recently new catalysts for direct methanol electrocatalysts have emerged for acidic electrolytes. Those catalysts have 10 to 100 times more activity than pure platinum with methanol and formaldehyde fuels. The conventional method at this time is to use the typical powder catalyst electrodes with no geometric differentiation of the location of the catalysts in the electrodes. The direct methanol reformer fuel cells have problems of creating produced carbon dioxide in the fuel supply that needs to be exhausted. The exhaust fuel stream is depleted of methanol and hydrogen by the fuel cell and is concentrated with carbon dioxide. The exhaust gas then usually is combusted to remove the residual hydrogen and is released to the atmosphere. A problem is that the release of hydrogen and methanol is an energy inefficiency of that fuel cell scheme, and the greater the carbon dioxide concentration of the fuel stream the lower the performance of the fuel cell. Ideally the exhaust from the fuel cell would have no unutilized methanol or hydrogen and be low in carbon dioxide. There would be no need for exhaust stream combustion.
Cell performances of 1 amp/cm.sup.2 with platinum catalysts on nickel substrates have also been achieved with alkaline electrolytes. Those experiments use platinum catalysts on porous nickel support structures. A current problem facing the alkaline cells is that the carbon dioxide generated from the fuel cell forms a carbonate precipitate in the electrolyte if the concentration of carbon dioxide in the electrolyte is sufficiently high.